Radiometric Measurements of Constants of Indicators. 



45 



1- 



A 



The wire loop was made of a very fine specimen of No. 36 copper 

 wire, furnished by Leeds and Northrup. Pure nitric acid was used to 

 dissolve the surface of the wire, which was very likely to be con- 

 taminated with various magnetic materials. That this wire must 

 have been very pure is shown by the behavior of the completed instru- 

 ment. As regards purity, a still better specimen of silver wire was 

 obtained from Weston; but unfortunately none of the junctions 

 attached to this silver wire was sufficiently sensitive for our purpose. 



The alloys used for the construction of the thermo-j unction were of 

 the composition recommended by Hutchins. 1 Figure 6 of the sketch 

 gives an enlarged view of the junction. It will be noticed that it 

 consists of two thermo-electric junctions A and B, which compensate 

 each other. When the same beam of light falls on both junctions, each 

 sends equal amounts of electrical energy 

 through the loop of non-magnetic wire w, 

 but in opposite directions. Hence, it follows 

 that for exact compensation there should be 

 no turn of the suspended system. In actual 

 operation the beam of light energy which is 

 to be measured is focused on one of the 

 junctions. This arrangement of junctions 

 is quite essential in any sensitive radio- 

 micrometer where a constant zero-point is 

 required; and it is only in this way that 

 the effects of temperature changes and stray 

 sources of light can be eliminated. 



The two arms represented by a and a' 

 in figure 6 have the composition 97 parts 

 bismuth and 3 parts antimony. The com- 

 position of b is 95 parts bismuth and 5 parts 

 tin. These metals are quite pure and the 

 strips used are very thin. Junction A is formed by sealing strip a to 

 strip b, and junction B is formed by sealing strip 6 to strip a'. R and 

 R' are the receiving surfaces, or so-called vanes, cut from very thin 

 tin foil, and soldered to junctions A and B, respectively. These 

 vanes each have an area of 4 sq. mm., and are coated with lampblack 

 to prevent radiation of light energy. It was found that the action of 

 an acidified solution of antimony chloride on the tin foil produces a 

 black receiving surface. Theoretically, it appears that such a metallic 

 receiving surface should be more effective than lampblack. However, 

 none of the junctions having the antimony receiving surface was 

 sufficiently sensitive to determine whether or not its use is to be pre- 

 ferred to that of lampblack. Greater sensibility can be obtained with 

 a single junction than with a compensating one, since in the former 



c' 



FIG. 6. 



'Sill. Journ., 48, 226 (1894). 



